da http://fy.chalmers.se/~appro/linux/DVD+RW/
It was observed that excessive reformats can render media unusable already after 10-20 reformats. It appears to be a firmware deficiency, not some common media defect [at least it was perfectly possible to salvage the media in a unit of different brand], but I don't recommend [enforced] reformat in either case. Note that DVD+RW re-formatting procedure does not substitute for blanking. If you want to nullify the media, e.g. for privacy reasons, do it explicitly with 'growisofs -Z /dev/scdN=/dev/zero'. Otherwise just write over previous recording as it simply wasn't there, no re-formatting is required.